Not Applicable
The present invention relates generally to a method and apparatus for directing or focusing dispersed charged particles into a low pressure apparatus. More specifically, the invention utilizes a jet disturber used in combination with an ion funnel to focus ions and other charged particles generated at or near atmospheric pressure into a relatively low pressure region, which allows increased conductance of ions and other charged particles into the device. The invention may further make use of a multi-capillary inlet to further enhance the conductance of such charged particles.
A great variety of scientific inquiry is confronted with the challenge of identifying the atomic structure or composition of particular substances. To assist in this identification, a variety of schemes have arisen which require the ionization of the particular substances of interest. Many of these analytical techniques, as well as the other industrial uses of charged particles, are carried out under conditions of high vacuum. However, many ion sources operate at or near atmospheric pressures. Thus, those skilled in the art are continually confronted with challenges associated with transporting ions and other charged particles generated at atmospheric or near atmospheric pressures into regions maintained under high vacuum.
An illustrative example of this general problem is presented in the use of electrospray ionization when combined with mass spectrometry as an analytical technique. Electrospray ion sources (which broadly includes, but is not limited to, nano electrosprays conventional electrosprays, micro-electrospray, and nebulizing gas assisted electrospray) are widely used with mass spectrometry for sample analysis, for example in biological research. For m/z analysis, ions are typically created at atmospheric pressure by the electrospray ion source and are then transported to the high vacuum region of a mass spectrometer through a capillary inlet that penetrates the first chamber of the mass spectrometer. A differential pumping system involving several stages for stepwise pressure reduction is commonly used to achieve the vacuum conditions conventionally utilized in m/z analysis within the mass spectrometer, and the major design issues are generally related to optimizing overall ion transmission efficiencies.
Improved transmission efficiencies in the intermediate vacuum stages have been achieved by using the recently developed RF ion funnel at higher interface pressures (xcx9c1 to 10 Torr) and RF multi-pole ion guides with buffer gas cooling at lower interface pressures as more fully described in Shaffer, S. A.; Tang, K.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D., Rapid Commun. Mass Spectrom. 1997, 11, 1813-1817; Shaffer, S. A.; Prior, D. C.; Anderson, G. A.; Udseth, H. R. and Smith, R. D. Anal. Chem. 1998, 70, 4111-4119; and Douglas, D. J.; French, J. B., J. Am. Soc. Mass Spectrom. 1992, 3, 398-408, and U.S. Pat. No. 6,107,628 entitled Method and Apparatus for Directing Ions and other Charged Particles Generated at Near Atmospheric Pressures into a Region under Vacuum, the entire contents of each of which are herein incorporated into this specification by this reference.
In co-pending U.S. patent application Ser. No. 09/860,727, filed May 18, 2001,
IMPROVED IONIZATION SOURCE UTILIZING A MULTI-CAPILLARY INLET AND METHOD OF OPERATION the entire contents of which are incorporated herein by this reference, a new interface having higher ion transmission efficiency compared to conventional interface designs is described. This interface, known as a multicapillary inlet, uses an array of capillaries to increase the gas throughput (i.e. the ion transmission) without sacrificing droplet desolvation efficiency and an electro-dynamic ion funnel for ion focusing into the next vacuum stage. To maintain the operating pressure of the ion funnel constant with the multi capillary inlet, the pumping for the first chamber (ion funnel chamber) is typically increased proportional to the conductance increase of the multicapillary inlet. It has been found that the directed gas stream from the larger conductance inlet was not completely dispersed, but retained some directed flow to the exit of the ion funnel. When the gas molecules with entrained ions enter into the first vacuum stage, the gas experiences an adiabatic expansion and forms a free jet. The expansion is surrounded by a concentric barrel shock and terminated by a perpendicular shock known as the Mach disc. In the expansion region, the gas molecules move in straight streamlines originating in the inlet. The region downstream of the Mach disc is known to have complex behavior. Far away from the inlet, the gas molecules move at random. There is a transition region where the directed motion changes into random motion in the region downstream of the Mach disc. In the ion funnel interface with the multi-capillary inlet, the transition region extends beyond the bottom of the ion funnel, and thus more gas is transferred to the second vacuum stage by the directed flow than with a single capillary inlet having a smaller conductance. Therefore, the pumping requirement in the second vacuum stage increases with the increase of the number of capillaries even though the ion funnel chamber is operated at the same pressure. Thus, an even higher vacuum pumping speed is required in the first stage (the ion funnel chamber) to maintain the second vacuum stage pressure in an acceptable range.
Thus, there exists a need for methods and apparatus that allow a reduction in the required pumping speed.
Accordingly, it is an object of the invention in one of its aspects to provide a method for providing an ion or charged particle source in a pressure region at near atmospheric pressures. As used herein, xe2x80x9cnear atmosphericxe2x80x9d pressures are defined as between 10xe2x88x921 millibar and 1 bar. Also as used herein, the charged particles are defined as being smaller than one billion AMUs. The focusing of the present invention is accomplished by providing an apparatus, hereinafter referred to as a xe2x80x9cjet disturberxe2x80x9d, which is positioned within an ion funnel. Most generally, a jet disturber may be any form of matter placed within the interior of an ion funnel that disperses the gas flow through the ion funnel. For example, in one preferred embodiment of the present invention, the jet disturber is simply a metal disc, mounted on a cross of two wires within the interior of an ion funnel perpendicular to the gas flow through the ion funnel. As will be recognized by those having skill in the art, a great variety of techniques and methods for placing an object within the interior of the ion funnel are possible, and any particular configuration that maintains any such object in such a manner should be construed as falling within the scope of the present invention.
While the present invention should be broadly construed to include any application wherein a jet disturber is used in conjunction with an ion funnel, it finds particular advantages when deployed to improve the ion transmission between an ESI source and the first vacuum stage of a mass spectrometer, and finds its greatest advantages when deployed in conjunction with a multicapillary inlet to introduce ions and other charged particles into a mass spectrometer. When deployed in this fashion, the jet disturber described herein has been demonstrated to provide greatly enhanced ion conductance.
These and other objects of the present invention are accomplished by providing a method for introducing charged particles into a device by first generating ions in a relatively high pressure region external to the device, directing the ions through at least one aperture extending into the device, and further directing the ions through an ion funnel within the interior of the device having a jet disturber positioned within said ion funnel. The present invention is most advantageously deployed when the aperture is provided as a multicapillary inlet, the relatively high pressure region is at between 10xe2x88x921 millibar and 1 bar, and the charged particles are generated with an electrospray ion source.
Accordingly, the method of the present invention is carried out with an apparatus for introducing charged particles generated at a relatively high pressure into a device maintained at a relatively low pressure comprising an ion funnel having a jet disturber positioned within said ion funnel. This apparatus is preferably interfaced with a multicapillary inlet extending into the device, whereby charged particles generated in the relatively high pressure region move through the multicapillary inlet and into the ion funnel.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.